1. Field of the Invention
This invention relates generally to overvoltage protection circuits and, more particularly, to an overvoltage protection circuit that has a thermal shut-down capability and a vertical power output thyristor fabricated in monolithic integrated circuit form.
2. Background Art
Previously known overvoltage protection circuits have been combined with a discrete thyristor to clamp the output voltage at a predetermined level. Furthermore, previously known overvoltage protection circuits have not included a means for shutting the circuit down when an overtemperature condition is reached.
One previously known voltage regulator, which is similar to an overvoltage protection circuit, is shown in FIG. 1 and includes a differential amplifier for comparing an input voltage with a bandgap reference voltage. A multi-collector PNP transistor has its base connected to the output of the differential amplifier and its emitter connected to a first supply voltage terminal. A first field effect transistor is coupled between a second supply voltage terminal and a first collector of the PNP transistor. A second collector of the PNP transistor is connected to the output voltage. The gate of the first field effect transistor is connected to the gate and source of a second field effect transistor, and is coupled to the first supply voltage terminal by a current source. A thermal limit circuit is coupled between the operational amplifier and the second supply voltage terminal for turning the operational amplifier off when a predetermined temperature is reached. This voltage regulator is described in greater detail in the Detailed Description of the Drawings.
However, the thermal limit circuit of this previously known voltage regulator would turn off a series power transistor instead of shunting the voltage to ground. Furthermore, the previously known overvoltage protection circuits require discrete power output thyristors.
Thus, a need exists for an improved overvoltage protection circuit that includes a power output thyristor fabricated in monolithic integrated circuit form and which further includes a thermal limit circuit for overtemperature protection.